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 Curve fitting with log y-axis and x linear axis

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T O P I C    R E V I E W
keerthanacivil Posted - 09/03/2021 : 02:37:20 AM
I have some data points which are plotted as x axis linear scale and y axis with log scale. On this plot, I need to do curve fitting whose equation is log(y/C)=exp[A*(x-B)], A,B,C are parameters. I created a new function. How to write this in the equation?

Kindly help me out
9   L A T E S T    R E P L I E S    (Newest First)
Sam Fang Posted - 09/06/2021 : 04:49:36 AM
Hi keerthanacivil,

I used your fitted parameters to calculate R2 and Adj R2, and found it is far less than 0.9. This R2 value comes from MATLAB or is calculated by you manually? Did you use a different method to calculate R2?

There are two methods to fit your data in Origin.
1. Transform your y data to log(y). And fit your data with the function (LabTalk script): y=log(C)+exp( A*(x-B) )

2. Fit y data directly, but choose Instrumental weight method, and select y as weight data. Here we use weight to force the curve to pass lower y points, otherwise the fit will ignore these points because lower y points contribute less to the residual sum of squares. And the function can be defined as: y=C*10^( exp(A*(x-B)) )

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keerthanacivil Posted - 09/03/2021 : 09:22:38 AM
I am attaching the data in an excel file. log(y/C)=exp[A*(x-B)] is the equation to be fitted in origin. I need confidence and prediction bands at 95% with color shading.
When I fitted using matlab, I got the parameters as, A=-0.242; B=11.06; C=1E-11, R2 as 0.91, Adj R2 as 0.91.
snowli Posted - 09/03/2021 : 08:57:11 AM
U can upload file when you reply the post. See the buttons above the big edit box when you reply. There is upload file button.

Thanks, Snow
keerthanacivil Posted - 09/03/2021 : 07:32:02 AM
To which mail I should send
keerthanacivil Posted - 09/03/2021 : 07:16:36 AM
Can you share the mail id
AmandaLu Posted - 09/03/2021 : 06:07:26 AM

Please send your data and fitting function to us so we can check:

You can find the fitting function file (.fdf file) in User Files Folder/ fitfunc folder.

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keerthanacivil Posted - 09/03/2021 : 06:01:11 AM
I tried using the parameter values obtained from matlab for fitting in origin. Still the fit is poor in origin. y = C*10^(exp(A*(x-B))) In this C is intercept and is same as the unit of Y-value, presented in log scale.
AmandaLu Posted - 09/03/2021 : 04:47:07 AM

Parameter’s initial value is very important for fitting with user-defined function. Please make sure you enter proper initial values.

OriginLab Technical Service

AmandaLu Posted - 09/03/2021 : 03:05:54 AM

Please refer to this page for how to define a fitting function in Origin:

To define your fitting function as explicit type, you will need to transform it to
y = C*10^(exp(A*(x-B)))

OriginLab Technical Service

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